netfilter: fix description of expected checkentry return code on xt_target

The text describing the return codes that are expected on calls to
checkentry() was incorrect.  Instead of returning true or false, or an error
code, it should return 0 or an error code.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>